1. Field of the Invention
The present invention relates to the conversion of polymers, particularly of polymer wastes into industrially useful products. More particularly, it relates to the conversion of mixtures of polymers into products of lower molecular weight and especially into light hydrocarbons which can be recycled or employed as raw material.
2. Background
Polymers are produced in continually growing quantities and most are employed in the single-use packaging sector.
However, the polymers' low propensity to natural decomposition, coupled with the decrease in the number of available tipping grounds or land fills, prompts the investigation of other routes for processing the used packages made of polymeric materials.
Incineration, a technique frequently employed for processing household waste, gives good results with polyolefins, whose complete combustion within the rules of the art gives water and carbon dioxide. Incineration is less suitable for other polymers which do not burn as well and tend to carbonize and produce noxious fumes.
Recycling of such polymers has had limited success because after collection, preliminary sorting according to type of polymer is required.
There is therefore a need for a large-scale polymer conversion process, particularly for mixed polymer wastes, into products of lower molecular weight and especially into lighter hydrocarbons which can be recycled or employed as raw material, for example for feeding oil refineries and petrochemical steam crackers.
The behavior of polymers at high temperature, if appropriate in the presence of suitable catalysts or reactants, has been investigated for many years and is well known to those skilled in the art. Depending on the circumstances, it results in a depolymerization (for example during the pyrolysis of polystyrene or the methanolysis or hydrolysis of polyethylene terephthalate) or in a decomposition (for example during the catalytic treatment of polyethylene).
Thus, U.S. Pat. No. 4,151,216 describes the catalytic cracking of polypropylene at 425.degree.-475.degree. C. on silica-alumina, giving a fuel stream which is liquid at 50.degree. C.
U.S. Pat. No. 4,143,086 discloses the catalytic cracking of amorphous polypropylene in a fluidized bed in the presence of a hydrocarbon feedstock, giving an effluent containing propylene.
European patent application No. 414 439 describes the conversion of high molecular weight polymers into products of lower molecular weight by heating the polymer (optionally dissolved) in contact with an acidic zeolite catalyst.
European patent application No. 276 081 and U.S. Pat. No. 4,584,421 describe two-stage polyolefin decomposition processes consisting of a thermal cracking followed by a catalytic cracking of the product of the first reaction.
U.S. Pat. No. 4,108,730 discloses a process for converting relatively ash-free solid polymeric wastes to more valuable liquid, solid, and gaseous products by mixing the polymeric waste at high temperatures in a refractory petroleum stream and catalytically cracking the mixture.
However, these processes do not envisage the treatment of mixtures of various polymers which may contain a high percentage of impurities, and the problems which arise therefrom. It therefore seems important and necessary in these times to have a simple process, comprising a minimum number of steps to treat polymer mixtures.
The Applicant has now developed an integrated process for the conversion of polymers, which process is particularly suitable for the conversion of polymer wastes into industrially useful products.